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This 3 page Class Notes was uploaded by Paxton Okuneva on Saturday September 19, 2015. The Class Notes belongs to CEN 4500 at University of Florida taught by Staff in Fall. Since its upload, it has received 16 views. For similar materials see /class/207041/cen-4500-university-of-florida in Computer Engineering at University of Florida.
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Date Created: 09/19/15
CEN 4500 Network Fundamentals Chapter5 The Network Layer The Network Layer Congestion Control Two Types of Congestion Control Open Loop Closed Loop Open Loop CC algorithms tends to be proactive that is they are designed to ensure that congestion does not occur in the rst place Tools used in Open Loop include Deciding when to accept new traffic Packetdiscard policies Scheduling decisions at various points along the network One Open Loop approach to congestion control is traffic shaping Used to offset the burstiness of data communications traf c shaping is about regulating the average rate of data transmission Works essentially by the customer and carrier agreeing beforehand what the customer s transmission pattern will be The carrier then monitors the customer s traf c to ensure the average throughput agreed is being maintained this is known as traffic policing Common Open Loop congestion algorithms are The Leaky Bucket Algorithm The Token Bucket Algorithm Note however that once a network connection is up and running no further congestioncorrections are made if Open Loop is utilized In contrast to Open Loop algorithms Closed Loop CC algorithms react to the network state in real time via the concept of a feedback loop This approach has 3 parts System monitoring for congestion Notification of entities that can affect the congestion Adjust system operation accordingly Two types of Closed Loop algorithms Explicit feedback lmplicit feedback With explicit feedback packets are sent from the point of congestion to warn the source that congestion is occurring With implicit algorithms the source deduces congestion by making local observations eg the time necessary to receive an acknowledgement Closed Loop Techniques Admission Control Choke Packets Weighted Fair Queuing Load Shedding Admission Control Very simple once congestion identified no more circuits allowed until congestion eases OR allow new circuits but route the new circuits around the problem areas Relevant only to virtual circuit connections Choke Packets Works on both virtual circuit and datagram subnets Each router monitors its output lines If it detects congestion a choke packet is sent back to the source The source then reduces it s output to the speci ed destination by a given Weighted Fair Queuing A router uses roundrobin on either a bytebybyte or packetbypacket basis to maximize throughput However certain hosttypes are allowed greater throughput per turn than others eg file servers are allowed twice the amount of throughput per turn than clients Load Shedding When previous methods fail routers can invoke load shedding simply dumping packets Not very elegant Least viable ofthe closed loop methods but may be necessary if congestion starts to get out of control Further Reading 39 Tanenbaum Chapter 53 539
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